In optical networks, chains of amplifiers and other network elements usually add spontaneous noise, thus degrading the optical signal which causes appearance of transmission errors. One of the main parameters of optical transmission—the required Optical Signal-to-Noise Ratio (OSNR) of a specific signal—depends on the initial signal power. The distance and number of amplifiers in the chain via which an optical signal is transmitted, also influence the overall OSNR and thus degrades the overall network performance. Standards in the field of optical communications (for example, ITU-T Standard Recommendation G.692, 10/98) refer to so-called pre-equalization for equalizing powers of optical signals which are degraded due to optical amplifiers' gain tilt. The pre-equalization partially compensates amplifier gain variation and gain tilt using the following scheme. The highest channel power in the system is assigned to the channel which will undergo the least line amplifier gain, whereas the lowest channel power is assigned to the channel that will undergo the most channel line amplifier gain. If pre-equalization is not used, the amount of channel power difference at the transmit interface leads to a reduction in the amount of amplifier gain variation and gain tilt which can be tolerated by the system.
U.S. Pat. No. 5,790,289 describes a wavelength division multiplexed (WDM) optical communication method and apparatus using a pre-emphasis technique to adjust the attenuation of a particular optical channel at a transmitter terminal to produce identical signal-to-noise ratios for all of the optical channels at a receiver terminal. The pre-emphasis adjustments to the transmitted signals are made on the basis of signal-to-noise ratio measurements performed at the receiver terminal. The signal-to noise ratio values for each channel are transmitted through a facing line that is also used to transmit data along optical communication lines from the receiver terminal back to the transmitter terminal.
U.S. Pat. No. 6,040,933 describes a method and apparatus for channel performance equalization in wavelength division multiplexed (WDM) systems. Performance of the channels is estimated from optical power measurements of each signal transmitted by the channels. The measurements are taken at the inputs of optical amplifiers in the transmission path of the system. The channels are equalized by adjusting the optical power of the channel transmitters. The method can compensate for signals having different bit rates by applying an offset to the amount of optical power adjustment of the channel transmitters. Furthermore, if different types of optical amplifiers are used in the transmission path, the method can accommodate different noise characteristics of the amplifiers by using their noise figures in determining the amount of optical power adjustment of the transmitters that is required to equalize channel performance.